Department of Biology, Indiana University, Bloomington, IN 47405, USA.
Proc Natl Acad Sci U S A. 2011 Nov 8;108(45):18542-7. doi: 10.1073/pnas.1107427108. Epub 2011 Oct 31.
The colorful process of chromatic acclimation allows many cyanobacteria to change their pigmentation in response to ambient light color changes. In red light, cells produce red-absorbing phycocyanin (PC), whereas in green light, green-absorbing phycoerythrin (PE) is made. Controlling these pigment levels increases fitness by optimizing photosynthetic activity in different light color environments. The light color sensory system controlling PC expression is well understood, but PE regulation has not been resolved. In the filamentous cyanobacterium Fremyella diplosiphon UTEX 481, two systems control PE synthesis in response to light color. The first is the Rca pathway, a two-component system controlled by a phytochrome-class photoreceptor, which transcriptionally represses cpeCDESTR (cpeC) expression during growth in red light. The second is the Cgi pathway, which has not been characterized. We determined that the Cgi system also regulates PE synthesis by repressing cpeC expression in red light, but acts posttranscriptionally, requiring the region upstream of the CpeC translation start codon. cpeC RNA stability was comparable in F. diplosiphon cells grown in red and green light, and a short transcript that included the 5' region of cpeC was detected, suggesting that the Cgi system operates by transcription attenuation. The roles of four predicted stem-loop structures within the 5' region of cpeC RNA were analyzed. The putative stem-loop 31 nucleotides upstream of the translation start site was required for Cgi system function. Thus, the Cgi system appears to be a unique type of signal transduction pathway in which the attenuation of cpeC transcription is regulated by light color.
色适应的多彩过程使许多蓝藻能够根据环境光颜色的变化改变它们的色素沉着。在红光中，细胞产生吸收红光的藻蓝蛋白（PC），而在绿光中，产生吸收绿光的藻红蛋白（PE）。通过在不同的光色环境中优化光合作用活性，控制这些色素水平可提高适应性。控制 PC 表达的光色感觉系统得到了很好的理解，但 PE 调节尚未解决。在丝状蓝藻 Fremyella diplosiphon UTEX 481 中，有两个系统控制 PE 的合成以响应光色。第一个是 Rca 途径，这是一个由光受体控制的双组分系统，在红光中生长时转录抑制 cpeCDESTR（cpeC）的表达。第二个是 Cgi 途径，尚未被描述。我们确定 Cgi 系统也通过在红光中抑制 cpeC 的表达来调节 PE 的合成，但作用于转录后，需要 CpeC 翻译起始密码子上游的区域。在红色和绿色光中生长的 Fremyella diplosiphon 细胞中，cpeC RNA 的稳定性相当，并且检测到包括 cpeC 5' 区域的短转录本，这表明 Cgi 系统通过转录衰减起作用。分析了 cpeC RNA 5' 区四个预测的茎环结构的作用。翻译起始位点上游 31 个核苷酸的假定茎环结构对于 Cgi 系统的功能是必需的。因此，Cgi 系统似乎是一种独特的信号转导途径，其 cpeC 转录的衰减受光色调节。
